Determination of cosmogenic Cl in rocks by isotope dilution: innovations, validation and error propagation. Desilets M. Zreda P. Almasi D. Measurements of cosmogenic Cl in terrestrial rocks provide quantitative information about exposure ages of landforms and surface features. The isotope dilution method for preparing Cl samples is now widely used because it allows Cl and Cl to be measured simultaneously on a single accelerator mass spectrometry target, increases the accuracy and precision of Cl determinations, and reduces rock sample size and laboratory work. In this paper we describe a new implementation of isotope dilution to Cl dating, report experimental data verifying the accuracy of this approach, and show how errors in the measured stable isotope ratio propagate to errors in exposure ages.
It is most useful for rocks which have been exposed for between 10 years and 30, years [ citation needed ]. The most common of these dating techniques is Cosmogenic radionuclide dating [ citation needed ].
Earth is constantly bombarded with primary cosmic rayshigh energy charged particles - mostly protons and alpha particles. These particles interact with atoms in atmospheric gases, producing a cascade of secondary particles that may in turn interact and reduce their energies in many reactions as they pass through the atmosphere. This cascade includes a small fraction of hadrons, including neutrons.
Measurements of cosmogenic Cl in terrestrial rocks provide quantitative information about exposure ages of landforms and surface features. The isotope dilution method for preparing Cl samples is now widely used because it allows Cl and Cl to be measured simultaneously on a single accelerator mass spectrometry target, increases the accuracy and precision of Cl Cited by: Measurement of 36 Cl in calcite, with an accelerator mass spectrometric detection limit of 5 ? 10 3 atoms per gram, allows dating of limestone surfaces exposed for periods ranging from 10 6 years. Alternatively, erosion rates from less than 1 to greater than ?m per year can be determined in the case of eroding karst surfaces. Chlorine has two stable isotopes and one cosmogenic isotope. The cosmogenic isotope, 36 Cl, has a long half-life, making it useful in age dating groundwaters up to 1 million years old. There is also limited variation in 37 Cl. Most natural variation in 37 Cl values in hydrologic systems are related to diffusion processes.
In rock and other materials of similar density, most of the cosmic ray flux is absorbed within the first meter of exposed material in reactions that produce new isotopes called cosmogenic nuclides. At Earth's surface most of these nuclides are produced by neutron spallation.
Using certain cosmogenic radionuclidesscientists can date how long a particular surface has been exposed, how long a certain piece of material has been buried, or how quickly a location or drainage basin is eroding. The cumulative flux of cosmic rays at a particular location can be affected by several factors, including elevation, geomagnetic latitude, the varying intensity of the Earth's magnetic fiel solar winds, and atmospheric shielding due to air pressure variations.
Rates of nuclide production must be estimated in order to date a rock sample. These rates are usually estimated empirically by comparing the concentration of nuclides produced in samples whose ages have been dated by other means, such as radiocarbon datingthermoluminescenceor optically stimulated luminescence.
The excess relative to natural abundance of cosmogenic nuclides in a rock sample is usually measured by means of accelerator mass spectrometry. Cosmogenic nuclides such as these are produced by chains of spallation reactions.
The production rate for a particular nuclide is a function of geomagnetic latitude, the amount of sky that can be seen from the point that is sampled, elevation, sample depth, and density of the material in which the sample is embedded.
Decay rates are given by the decay constants of the nuclides. These equations can be combined to give the total concentration of cosmogenic radionuclides in a sample as a function of age.
The two most frequently measured cosmogenic nuclides are beryllium and aluminum These nuclides are particularly useful to geologists because they are produced when cosmic rays strike oxygen and siliconrespectively. The parent isotopes are the most abundant of these elements, and are common in crustal material, whereas the radioactive daughter nuclei are not commonly produced by other processes.
As oxygen is also common in the atmosphere, the contribution to the beryllium concentration from material deposited rather than created in situ must be taken into account.
Surface exposure dating is a collection of geochronological techniques for estimating the length of time that a rock has been exposed at or near Earth's surface. Surface exposure dating is used to date glacial advances and retreats, erosion history, lava flows, meteorite impacts, rock slides, fault scarps, cave development, and other geological events. Apr 01, We report the result of a pilot study designed to establish the absolute age of a large postglacial rotational rockslide at The Storr on the Isle of Skye, Scotland, using 36 Cl surface exposure dating. Exposure ages of ± cal. ka BP and ± cal. ka BP were obtained for rock samples from two separate landslide blocks, giving an Cited by: Nov 25, Chlorine dating and the bluestones of Stonehenge by Olwen Williams-Thorpe, D. Graham Jenkins, Judith Jenkins, John S. Watson Chlorine dating has important potential for archaeology, but recent Chlorine dates on 'bluestones' of .
Zreda; et al. Earth and Planetary Science Letters.
Sheppard and M. Herod Journal of Environmental Radioactivity. Archived from the original on Categories : Isotopes of chlorine Environmental isotopes. Hidden categories: Isotope content page.
Chlorine dating is generally applicable to water in the age range ka to 1 Ma. Chlorine in the Hydrological Cycle The principal application of 36Cl to the dating of old groundw ater is by measurement of the decay of atmospheric 36Cl. Chlorine is produced in the atmosphere by cosmic-ray spallation of 40Ar (Lal and Peters, ).File Size: 1MB. In situ cosmogenic 36 Cl is produced by interaction of cosmic rays with different target elements, mainly calcium, potassium and chlorine (35 Cl). 36 Cl can chemically be extracted from any kind of rock or mineral that contains at least one of these target elements, such as limestone, Ca-/K-feldspar or Ca-pyroxene. We try to avoid rocks with high chlorine concentrations because of .
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